The treatment of band-shaped or strip-shaped goods, for example strip-shaped thermoplastic or metallic films or foils, is carried out, among other things, with equipments or installations having rotationally driven goods guiding rollers. Such goods guiding rollers cooperate with rotationally driven pressure rollers, on the one hand especially to prevent air from becoming entrapped between the goods guiding rollers and the strip-shaped goods, and on the other hand to obtain a sufficient friction between the roller and the foil or film. With a thermoplastic film, for example, air inclusions or entrapments between the goods guiding roller and the pressure roller are avoided in that a pressure roller equipped with a rubberized roller jacket or sheath is pressed on the linear forward end or take-up of the film onto the goods guiding roller.
The pressing must be carried out linearly and with a nearly constant pressure level of the pressure roller over the width of the film, and particularly on the forward end or take-up of the goods guiding roller. With a relatively large spacing distance of the bearings taking up the rollers, it is complicated to satisfy this abovementioned requirement, because on the one hand the flexing or bending deflection of the roller increases with increasing spacing distance of the bearings, and on the other hand the applied pressing force leads to the result that the linear contact of the rollers over the width of the goods is lost, that is to say the rollers only still have touching contact in the area of their ends, while no pressing force is achieved in the middle area. A further increase of the pressing force leads to no better result, more likely to a further increase of the gap between the rollers. In the operation with a small pressing force distributed uniformly over the roller width, it was determined that in this pressure range, the friction of the piston packing seals or sleeves in a pneumatic piston-cylinder unit prevents an exact adjustment of the pressing force of the pressure roller onto the goods guiding roller. Upon increasing the working pressure the friction forces of the piston-cylinder unit would be overcome, but however, simultaneously an undesirably high pressing force would be realized, which leads to the above mentioned deficiencies.
From the German patent publication DE 103 44 710 of the same assignee as the present US application, there is known a method and an apparatus for the regulation of the pressing force of a pressure roller onto a goods guiding roller, with which a prescribed nominal or rated pressing force of a pressure roller that can be applied onto a goods guiding roller with at least one pneumatically driven piston-cylinder unit is not exceeded. This prior invention of DE 103 44 710 remedies the abovementioned disadvantages.
In practice it has been shown, however, that in addition to the regulatable pressing force of the applied pressure roller, also the attainment of the touching contact between the rollers and the film has a decisive influence on the quality of the film. In order to avoid disadvantageous effects on the quality of the film in connection with the touching contact, before being applied the pressure roller is driven with a circumferential velocity that corresponds to the film velocity. Furthermore it is advantageous if a linear touching contact simultaneously arises on the entire roller width between the rollers and the film. This requirement is, however, difficult to satisfy, due to the typical dimensions in the field of the film stretching equipments or machines and the correspondingly large rollers with correspondingly large masses, because one must work with large pneumatically driven piston-cylinder units. Typically in this regard, rollers with a mass of, for example, 1800 kg are used. For moving this mass a certain minimum diameter of the piston-cylinder units is required. For safety reasons and for easier pulling-in of the goods, a certain minimum spacing distance between the rollers in the retracted position is furthermore required. This minimum spacing distance must be overcome during the applying of the pressure roller and thus necessitates a certain minimum stroke of the piston-cylinder units. Sufficiently dimensioned pneumatic piston-cylinder units, however, do not carry out exactly uniform movements due to the friction of the piston packing seals or sleeves, and have a long actuation time due to the large volume and limited air volume flows. The abovementioned invention can therefore not remedy the disadvantage of the non-uniform applying of the pressure roller onto the goods guiding roller.